EFFECT OF EARLY SOLVENT EVAPORATION ON THE MECHANISM OF THE SPIN-COATING OF POLYMERIC SOLUTIONS

The effect of solvent evaporation on spun‐on polymer thickness is investigated. Film thickness is shown to be proportional to spin speed raised to the negative of a constant, b. This constant is specifically considered. Of particular interest is the short initial period during spin coating signified by a rapid increase in solution viscosity, but where solvent diffusion in solution is not yet significant. Many previous theories have suggested the existence of a single value for b dependent only upon environment and spin coating system. Here, a single polymer dissolved in five pure solvents is examined. The polymer is Thermid IP‐600 (National Starch and Chemical Co.), an isoimide precursor. The solvents considered are n‐methyl‐2‐pyrrolidinone, dimethyl sulfoxide, dimethyl formamide, ethylene glycol dimethyl ether, and 2‐methoxyethyl ether. Spin Speed ranges from 0.6 to 7.0 kRPM and spin time is held at 15 s. It is shown that b varies from 0.97 for n‐methyl‐2‐pyrrolidinone to 0.57 for 2‐tncthoxycthyl ether. An empirical relation describing the behavior of b for this particular polymer and spin conditions used is b = [(MW)s(Pvap)s]/[(MW)n(Pvap)n]−0.1, where (MW)s and (MW)n are the molecular weights of the solvent and of n‐butyl acetate, respectively and where (PVaP)s and (Pvap)n are the vapor pressure of the pure solvent and the vapor pressure of n‐butyl acetate, respectively. Copyright © 1990 Society of Plastics Engineers